Tilt the ball head mount into the notch, Arca grip w vertical orientation.
The Sidekick base (pin) slides down in to the Arca grip (tail) on the
(Kirk BH-3) ball head. Like anything else in the Arca grip, you tighten it down.
Then you lock down the ball itself; no benefit to any movement there..
(The top of the Sidekick base has a stop that rests against the Arca grip.)